The present invention relates to a device for generating a holographic reconstruction of a three-dimensional scene, which comprises a number of objects. To be able to compute and to encode holograms of the scene, the scene is divided into individual object points, which are combined to form object point groups that are represented in a light modulator means in the form of computer-generated holograms (CGH). Using coherent light and a reconstruction means, individual reconstructions are generated of each object point group and superposed so that an observer sees from an eye position the temporally averaged scene with reduced speckle patterns. The present invention further relates to a method for generating a holographic reconstruction of a scene such to allow speckle patterns to be reduced.
This invention can be applied in conjunction with devices which allow complex wave fronts of a three-dimensional scene to be recorded and reconstructed with the help of holography using coherent laser light in real-time or in near-real-time, where the reconstruction can be seen from a visibility region, which is also called an observer window. A light modulator means with controllable elements is provided for modulating the wave fronts of the incident coherent light with the complex values of the scene.
A particular type of a holographic display in which the novel method can be applied is known from earlier documents filed by the applicant, e.g. from (1) EP 1 563 346 A2, (2) DE 10 2004 063 838 A1 or (3) DE 10 2005 023 743 A1. There, the hologram computation is executed on the following basis: For encoding and holographic reconstruction, a three-dimensional scene is sliced into section layers, each of which comprising a multitude of object points of the scene. The object points characterise both the surfaces and, as the sum of all surfaces, the three-dimensional scene. They are written as complex values to (or encoded in) multiple controllable elements of the light modulator means, each object point thus forming a separate region on the light modulator means. Such a separate region is referred to as the sub-hologram of this object point. The sub-hologram corresponds roughly to a holographically encoded lens function which reconstructs this one object point in its focal point. The absolute value of the complex values, i.e. the amplitude, is about constant across the entire sub-hologram, and its magnitude depends on the axial distance of the object point to the screen and on the intensity of the object point. The phase distribution of the complex values in the sub-hologram corresponds roughly to the function of a lens whose focal length depends on the axial distance of the object point to the light modulator means or screen. As coherent light passes through the light modulator, the complex values which are written to the controllable elements of the sub-hologram modify the amplitude and/or phase of the light. The object point can be reconstructed by the modulated light. Outside the sub-hologram, this object point has the value ‘0’ in the light modulator means. The total encoded hologram of the scene is generated by adding the complex values of the individual sub-holograms.
The holographic reconstruction of the scene is generated by a reconstruction means in a reconstruction space which stretches between the visibility region and the light modulator means. The wave fronts which are emitted by the encoded holograms of the scene are superposed in the visibility region, so that the reconstructed object points can be seen there from an eye position. The reconstruction is generated based on the superposed wave fronts in that individual perspective views of the scene are generated for each eye of an observer in a time- or space-division multiplex process, where said views differ in parallax, but are perceived by the brain as a single holographic 3D representation.
For watching the reconstruction of the three-dimensional scene, the observer can either look at a light modulator means on which a hologram of the scene is directly encoded, and which serves as a screen. This is referred to as a direct-view arrangement. Alternatively, the observer can look at a screen onto which either an image or a transform of the hologram values which are encoded on the light modulator means is projected. This is referred to as a projector arrangement. The eye positions of observers are detected by a position finder in a known manner, said position finder being linked by software means with a storage means and a computing unit, and with a system controller means. The storage means also hosts the information of the object points which are necessary for computing the CGH in data records in the form of a look-up table.
Because the light modulator means only allows discrete recording, the object points of the scene are scanned discretely for hologram computation. Certain encoding methods provide the possibility to generate a reconstruction which fully corresponds with the scanned scene at the position of the scanning points. However, the physical reconstruction results in a continuous gradient of the reconstructed intensity, also between the scanning points. These positions show deviations from the intensity gradients in the scene, which cause the speckle patterns in the reconstruction and which thus deteriorate the quality of the holographic representation. This is in particular the case when the hologram is computed with a random phase of the object points. Generally, a speckle pattern can be described as a granulation-like interference pattern which is created as a spatial structure with randomly distributed intensity minima and maxima by interference of multiple light waves with statistically irregularly distributed phase differences. These speckle patterns substantially deteriorate the quality of the perception of the reconstructed scene.
Speckle patterns can generally be reduced by temporal and/or spatial averaging during the reconstruction of the three-dimensional scene. The observer eye always averages out multiple reconstructions presented to it, where each of these reconstructions has a different speckle pattern. The speckle pattern will for example be random and different if the object points of the scene exhibit different random phases. Thanks to the averaging effect, the observer perceives a minimisation of this speckle pattern. Temporal averaging with the aim to reduce speckle patterns is for example described by Donghyun Kim in the document “Reduction of coherent artifacts in dynamic holographic three-dimensional displays by diffraction-specific pseudorandom diffusion”. Different holograms of a scene are computed and displayed one after another, where the individual object points of the scene are superposed with varying relative phase differences. The eyes thus temporally average away the interference effects, i.e. the speckle patterns. However, one has to accept greater computational load caused by the need to compute multiple holograms with this method, because each hologram is always computed for all object points. This can be a substantial drawback in real-time representation of reconstructed scenes. Moreover, inexpensive light modulator means with shorter switching times are required for the holographic representation. Such devices are not yet commercially available.
Further, the resolving capacity of the human eye must be taken into consideration when reconstructing a scene in a holographic display. In order to ensure that planar surfaces in a scene are perceived as continuous planes by an observer, and not as a collection of individual points, a critical distance between adjacent object points must not be exceeded within this plane or section layer when arithmetically dividing the scene into object points. However, in particular those interferences which occur between object points which lie close to each other contribute the major share to the speckle patterns which need to be eliminated.